With increasing use of fluidized bed combustion techniques for fossil fuels and in chemical and metallurgical processes, the development of apparatus capable of removing dust from the gaseous reaction products, generally the elevated temperatures and pressures, has been of increasing interest in recent years.
For filtration at temperatures of 500 to 1200.degree. C. and higher, so called candle filters have been developed utilizing tilter "candles" or tubes composed of porous ceramic material.
Such filter tubes cannot, at the present time, be fabricated with unlimited lengths or even significant lengths at will. In practice it is found that the longest length of filter tube of such refractory ceramic filters is about 1.50 m. To obtain the requisite filter area, therefore, a large number of filter tubes must be mounted in a housing of a limited footprint so as to occupy a minimum space in the plant. As a consequence, the filter candles must be arrayed in a plurality of stages, one above another.
The present invention is intended to develop the apparatus described in European Patent Document EP-B1-0129053. This apparatus comprises an upright cylindrical housing in which clean gas collecting assemblies are disposed, each of which comprises a circular planar bottom plane and a small annular shell or lateral wall surmounted by a conical roof. Between the lateral shell and the housing wall, a small gap is provided for the downwardly directed gas stream. The filter elements are suspended from the perforated bottom plate. The clean gas collecting assembly is connected with a coaxial clean gas pipe that can pass either through the roof or through the bottom of the housing which may be formed as a dust discharging funnel.
To supply a drive gas for the backflushing of the filter surface and dislodging the solids deposits on the outer surface of the candle-type filter elements, each stage has a horizontal blast pipe extending through the shell portion of the respective clean gas collecting assembly. Each blast pipe is juxtaposed with a group of a multiplicity of filter elements and is provided with downwardly directed nozzles each aligned above a respective filter element.
In this apparatus, the individual filter elements are traversed by the gas stream to be cleaned generally in a cross flow. Upon a cleaning of the filter by the blast from the blast pipe, there is a tendency for dislodged dust, especially of the finer particle size fraction, to be entrained onto the filter element.
This results in a shortening of the operating internal of the filter and an increase in the cleaning time. In many cases the filter element can become irreversibly contaminated with fine particles, thereby reducing the useful life of the filter elements.
During the cleaning of a filter element group, the dislodged dust tends to fall upon the conical roof of the assembly disposed below the unit being cleaned.
If the angle of the roof is not sufficient to cause the dust to pass downwardly directly, additional cleaning devices are required to dislodge the dust from the roof. An alternative, of course, is to increase the angle so that it is at least equal to the friction angle, i.e. the angle at which the dust will slide autogenously from the roof. However, the lengths of the filter elements must fall off in the direction of the housing axis in accordance with the steepness of the roof. This has a detrimental effect on the overall filter area for a housing of a given size.
The long branched pipes utilized for supplying the drive gas under high pressure to the chamber traversed by the hot gas subjects the pipes to thermal stress and, when these pipes are composed of austenitic steel that may have only a limited strength, pressure shocks and the reaction forces at elbows and turns of the pipe can result in damage to it.
In German Patent Document DE-AS 17 57 635, a filter apparatus is described which has a raw gas inlet at an upper part of the housing and a single clean gas collecting unit which is located in a lower part of the housing directly above a dust collecting funnel. It comprises a plurality of parallel collecting ducts forming a kind of grate with intervening spaces. On the upper sides of these collecting ducts, filter hoses are connected which, in turn, are suspended from an upper part of the housing. The individual collecting ducts traverse the housing wall and open into a separate clean gas chamber. Each collecting duct has at its end a nozzle formation into which a drive gas nozzle projects. The ends of the collecting ducts are connected with an external gas pipe.
In this apparatus, the raw gas flows in a vertical direction along the filter hoses. When one group of filter hoses is to be cleaned, the raw gas stream promotes the downward fall of the released dust. The dust passes through the gaps between the collecting ducts into the dust funnel. A gas stream does not flow through these gaps.
The apparatus does not appear to be suitable for the cleaning of hot gas, not only because the use of filter hoses is not amenable to the development of high temperatures, but also because of the arrangement of the collecting ducts. The fact that the collecting ducts pass through the housing walls means that thermal stresses are generated which cannot be readily withstood by the materials of the ducts and wall if high temperatures are used. The flushing with external gas, furthermore, would result in a thermal shock which could only be avoided by heating the external gas to the temperature of the clean gas at additional cost and complexity.